The invention relates to a cavity down seam welding ceramic package for sealing a semiconductor device.
In the prior art, a cavity down ceramic package for a semiconductor device has utilized a solder sealing of a metal cap using an Au--Su soldering agent. However, in view of the low cost and a low temperature sealing as well as a reduction of pin holes of soldering being recently required, it has been on the consideration to use a seam welding for sealing the cavity down mounting ceramic package.
In FIGS. 1A and 1B, the conventional seam welding method for the ceramic package with a circular seal ring is illustrated, while in FIGS. 2A and 2B, the conventional seam welding method for the ceramic package with a square-shaped seal ring is illustrated. A ceramic package 1 has a cavity portion la at a center portion on a top surface thereof for mounting a semiconductor device. Outer leads 2 are provided in the form of a lattice with a pitch of 2.54 millimeters on a peripheral area of the top surface of the ceramic package 1. In the cavity portion 1a, a semiconductor device 3 is mounted. The semiconductor device has pads being connected to inner leads through bonding wires 7. The inner leads are connected to the outer leads 2 through interconnections.
On the peripheral area to the cavity portion 1a of the semiconductor device 3, a metallization layer is formed. A seal ring 4 made of Fe--Ni alloy or Kovar is brazed by Ag--Cu brasses on the metallization layer. The seal ring has a thickness of 0.5 millimeters and a width of 1 to 1.5 millimeters and a surface thereof is treated by Ni and Au gilding. A metal cap 5 is fixed on the seal ring 4 by the seam welding.
The metal cap 5 is made by punching of Ni gliding or Ni cladding Kovar material. The metal cap has a concave center portion in the seal ring 4 and a peripheral portion on the seal ring 4. The seam welding is carried out by rotation or motion of paired roller electrodes 6 in contact with the peripheral portion of the metal cap along the seal ring 4 during which a voltage of 3 V to 4 V and pulse currents of approximately 200 mA are applied to the roller electrodes 6. In case of the circular metal cap 5, the package is rotated one or half time around a center axis. In case of the square shaped metal cap 5, the package or the roller electrodes are moved in the straight and in relation to each other so that the roller electrodes 6 shows a motion with a rotation along first opposite sides of the square shaped metal cap 5 for subsequent rotation of the package by 90.degree. and then the roller electrodes 6 rotates along second opposite sides of the square shaped metal cap 5. If the square metal cap 5 has rounded corners with a large curvature radius then it is difficult to cause a sufficient metal melting necessary for a secure sealing. For that reason, the curvature radius of the corners of the square metal cap 5 is preferably limited within 0.5 millimeters.
The above described rotary and parallel seam welding methods are engaged with the following problems in the rotary seam welding method as illustrated in FIGS. 1A and 1B, a size of the semiconductor device is largely limited by an external definition of the package 1 and a placement of the outer leads 2. A mounting area for the semiconductor device when use of the circular metal cap is smaller by 20 to 30 percent than that when use of the square metal cap. This means that the package with the circular metal cap has a larger unavailable area.
In the parallel seam welding method as illustrated in FIGS. 2A and 2B, it is required to provide dead zones on which no outer leads are provided for the motion of the roller electrodes 6 for the seam welding when the roller electrodes 6 is positioned on the corners of the square metal cap, then a half of each the roller electrode 6 extended around or outside of the square metal cap. For that reason, it is difficult to provide the outer leads 2 on the dead zones in the vicinity of the corners of the square metal cap, thereby resulting in the number of the outer leads being largely limited. When the roller electrode 6 has a width of 4 millimeter and a radius of 12 millimeter and a pitch of the outer leads is 2.54 millimeter, each dead zone may provide an elimination of 2.times.2 outer leads. Two of the dead zone are formed in the vicinity of each corner of the square metal cap. Then, eight of the dead zones are formed in the vicinity of the corners of the square metal cap thereby eliminating 32 outer leads.